Prior, the quality issue of sand and gravel aggregates is a crucial problem in the raw materials of concrete. With a vast variety of sand and gravel aggregates and numerous irregularities, it severely affects the safety of concrete quality.
Ultimately, the scarcity of sand and gravel aggregates is just one aspect; the main reason lies in the managers' lack of attention to the quality of sand and gravel aggregates, or excessive interference by concrete enterprise managers in the aggregates, with the "heavy cement, light aggregates" mindset deeply ingrained.
Currently, the issue of aggregate quality is a crucial problem in the raw materials of concrete, with a wide variety of aggregates and a proliferation of chaos, severely affecting the safety of concrete quality.
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I. The Main Indicators of Quality Issues with Sand and Gravel Aggregate
(1) The gravel contains a high amount of mud. Currently, many regions have banned the mining of river sand, and even where there is a supply, the quality is poor due to a high mud content. Many concrete companies use stone debris, also known as stone chips, as fine aggregates, and sometimes the mud content in the stone powder is astonishing.
(2) Poor aggregate gradation. This is primarily evident in stone factories that sell stones graded into sizes of 5-10mm, 10-25mm, and 20-31.5mm, respectively, leading to poor gradation when used individually by concrete companies. Additionally, the small corner pieces of stone dust from stone factories are often sold to concrete companies as so-called "mechanical sand." The gradation of the stone dust usually features a high content of particles at both ends and insufficient particles in the middle, posing difficulties in concrete mixing.
(3) Fine aggregate sand has a wide variation in fineness modulus. Due to different sources and materials, the fineness modulus varies greatly. Concrete companies often have multiple suppliers for fine aggregates, leading to cross-deliveries that exacerbate the fluctuations in the fineness modulus of fine aggregates.
(4) Poor aggregate quality. The aggregates contain a high amount of weathered, soft particles, leading to low crushing value and high water absorption rate. This, on one hand, deteriorates the workability of the concrete mixture, and on the other hand, results in lower concrete strength and reduced durability.
(5) Poor aggregate particle shape. Affected by the parent rock material and production process of the aggregates, the content of needle-like and flake-shaped particles is excessively high. Some particles may not meet the specifications of needle-like and flake-like shapes, but their irregular shapes lead to reduced workability and mechanical properties of the concrete.
Strengthening sand and gravel testing is crucial for controlling aggregate quality.
The initial assessment of the quality of fine aggregates is conducted using a "experience and testing" method, with experience as a supplement and testing as the primary approach.
The testing experience method for fine aggregates involves "observing, pinching, rubbing, throwing, and washing."
"Observe the gradation, estimating the coarseness level. Grasp a handful of sand and spread it in your palm, examining whether the fine and coarse grains are evenly distributed. The more evenly the gradation of particles is distributed, the better the gradation."
"Pinch," grab a handful of sand to estimate its moisture content. Observe the sand ball's consistency after pinching; the tighter the sand ball, the higher the moisture content, and conversely, the lower it is.
"Scrape a handful of sand into the palm of your hand, rub both palms together, then gently clap your hands. Observe the layer of mud adhering to the palm: the thicker and more yellow the mud, the higher the mud content in the sand; conversely, a lower mud content."
"Toss" the sand into a ball in your palm and give it a toss. If the sand ball remains intact without falling apart, it can be determined that the sand is fine-grained, contains a high amount of silt, or has a high water content.
"Rinse a handful of sand in water, observe the degree of murkiness to determine the mud content, and also inspect the material and grain shape of the sand particles after washing."
The experience in testing coarse aggregates primarily relies on intuitive methods such as "inspecting and grinding."
"Observe the granule gradation, inspect the particle shape of the coarse aggregates, examine surface impurities and silt content, and assess the amount of weathered soft particles."
"Grind" refers to the process of rubbing two or more coarse aggregate particles together in the hand to determine the hardness of the coarse aggregate.
If there is any doubt about the quality of the aggregate determined initially, it should be immediately tested to verify, in order to conduct a quantitative analysis.
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